DE102014201345A1 - On-board network and method for operating a vehicle electrical system - Google Patents

Abstract

The invention relates to a vehicle electrical system (1) for a vehicle having a low-voltage sub-network (21) for at least one low-voltage consumer (29) and having a high-voltage sub-network (20) for at least one high-voltage consumer (25) and an electric generator (23), wherein the high-voltage sub-network ( 20) comprises a battery (40) which is adapted to generate the high voltage and output to the high voltage subnetwork (20) and the at least two battery units (41) with Einzelspannungsabgriffen (42), wherein the high voltage subnet (20) with the low voltage subnet ( 21) is connected via a coupling unit (33) which is set up to take energy from the high-voltage sub-network (20) and to supply it to the low-voltage sub-network (21). It is provided that the coupling unit (33) is arranged to selectively connect the battery units (41) to the low-voltage sub-network (21). The invention also relates to a method for operating a vehicle electrical system, a motor vehicle, as well as a battery management system and a computer program, which are set up for carrying out the method.

Description

State of the art

The invention relates to a vehicle electrical system and a method for operating a vehicle electrical system for a motor vehicle.

Furthermore, a motor vehicle is specified with such a vehicle electrical system, and a battery management system and a computer program, which are set up to carry out the described method.

In motor vehicles with an internal combustion engine, an electrical system is provided for supplying the electric starter or starter for the internal combustion engine and other electrical devices of the motor vehicle, which is operated by default with 12 volts. When the internal combustion engine is started, a voltage is made available to a starter via the electrical system of a starter battery, which starts the internal combustion engine when, for example, a switch is closed by a corresponding starter signal. If the internal combustion engine is started, this drives an electric generator, which then generates a voltage of about 12 volts and provides it via the electrical system to the various electrical consumers in the vehicle. The electric generator also charges the starter battery charged by the starting process. If the battery is charged via the electrical system, the actual voltage may also be above the rated voltage, eg. B. at 14 V or at 14.4 V. The electrical system with 12 V, or 14 V voltage is referred to in the present disclosure as a low-voltage electrical system.

It is known to use in electrical and hybrid vehicles, another electrical system with a nominal voltage of 48 V, which is referred to in the context of the invention as a high voltage electrical system.

Disclosure of the invention

According to the invention, a vehicle electrical system for a motor vehicle comprises a low-voltage sub-network for at least one low-voltage consumer and a high-voltage sub-network for at least one high-voltage consumer and an electric generator, wherein the high-voltage subnet is connected to the low voltage subnet via a coupling unit which is adapted to take energy from the high voltage subnet and supply the low voltage subnet wherein the high voltage subnet comprises a battery configured to generate and output the high voltage to the high voltage subnet and having the at least two battery units with single voltage taps routed to the coupling unit. It is provided that the coupling unit is set up to selectively connect the battery units to the low-voltage subnet.

The invention has the advantage that can be operated by the low voltage subnet electrical consumers, which are designed for a low first voltage, and for high-power consumers, the high-voltage sub-network is ready, i. the sub-board network with the voltage compared to the first voltage increased. The supply of the low voltage subnetwork is superimposed on the charging and discharging processes in the high voltage subnetwork. The low voltage subnetwork supply via the high voltage subnetwork takes place unidirectionally, d. H. The coupling unit preferably provides the energy transfer only in one direction.

The terms "battery" and "battery unit" are used in the present description, adapted to common usage, used for accumulator or Akkumulatoreinheit. The battery includes one or more battery units that may designate a battery cell, a battery module, a module string, or a battery pack. The battery cells are preferably spatially combined and interconnected circuitry, for example, connected in series or parallel to modules. Several modules can form so-called battery direct converters (BDCs), and several battery direct converters form a battery direct inverter (BDI).

The electrical system can be used both in stationary applications, e.g. in wind turbines, as well as in vehicles, e.g. in hybrid and electric vehicles.

In particular, the electrical system can be used in vehicles that have start-stop systems.

The presented system, d. H. the electrical system and the battery management system is particularly suitable for use in vehicles having a 48 volt generator and a 14 volt starter, the 14 volt starter is preferably designed for start / stop systems.

The system presented is particularly suitable for use in vehicles that have a boost recuperation system (BRS). Boost recuperation systems (BRS) generate electrical energy during braking, downhill or sail operation to supply the electrical consumers. The BRS increases the efficiency of the system so that fuel can be saved or emissions can be reduced can. The battery in the high-voltage subnetwork either supports the internal combustion engine, which is referred to as boost, or it is even used at low speeds for short distances for purely electric driving, for example in an electric parking and parking.

Advantageous developments and improvements of the subject matter specified in the independent claim are possible by the measures listed in the dependent claims.

It is advantageous if the selectively switchable battery units are each designed to provide the low voltage. The battery units can thus be alternately claimed to provide the low voltage, z. B. to support a start-stop system, resulting in an increased life of the battery unit.

According to a preferred embodiment, the coupling unit has at least one reverse-blocking switch. The reverse blocking switches are preferably suitable for connecting and disconnecting a selectively connectable battery unit. These switches have the property that in the "on" state, they allow current to flow in only one direction and, in the "off" state, they can absorb a blocking voltage of either polarity.

According to a preferred embodiment, the low-voltage subnetwork has at least one energy store, which is set up to generate the low-voltage and to output it to the low-voltage subnet. The energy store can also be designed as a double-layer capacitor. Since, according to some embodiments, the low-voltage subnetwork can be operated solely with the battery units of the high-voltage subnetwork, the energy store arranged in the low-voltage subnetwork is, in principle, redundant for the vehicle electrical system. However, the use of the energy store arranged in the low-voltage subnetwork is particularly advantageous when the battery units of the high-voltage subnetwork comprise lithium-ion batteries. Lithium-ion batteries have the disadvantage that they can not provide the optimum performance in certain applications or situations, in particular, for example, at low temperatures. The energy store arranged in the low-voltage subnetwork compensates for this, so that a system which is stable even in extreme situations is provided.

gewählt, wobei I_max der maximale Bordnetzstrom ist, der während Umschaltvorgängen im Niederspannungsteilnetz fließen soll, t_umschalt die Zeitdauer, während der keine Batterieeinheit für die Versorgung bereitsteht, und ΔU_max die maximal zulässige Veränderung der Bordnetzspannung während des Umschaltvorgangs. Der Kondensator eignet sich außerdem bevorzugt auch als Energiespeicher, welcher eingerichtet ist, zumindest kurzfristig die Niederspannung zu erzeugen und an das Niederspannungsteilnetz auszugeben.According to a preferred embodiment, the low-voltage subnetwork has at least one capacitor. The capacitor is preferably configured to stabilize the low voltage when changing the connected battery unit. The dimensioning of the capacitor is preferred according to

selected, where I _{max is} the maximum on-board electrical system current that should flow during switching operations in the low voltage subnet, t _switch the time during which no battery unit is ready for the supply, and ΔU _max the maximum allowable change in the vehicle electrical system voltage during the switching process. The capacitor is also preferably also suitable as an energy store, which is set up, at least in the short term to generate the low voltage and output to the low voltage subnet.

According to a preferred embodiment, the low-voltage subnet has a starter. The starter is preferably designed for start / stop systems and configured to actuate a switch for starting an internal combustion engine.

In a method according to the invention for operating a vehicle electrical system for a motor vehicle, the vehicle electrical system having a low voltage subnet for at least one low voltage consumer and a high voltage subnet for at least one high voltage consumer and an electric generator, wherein the high voltage subnet is connected to the low voltage subnet via a coupling unit which is arranged the energy to be taken from the high-voltage sub-network and the low voltage subnet, the high-voltage subnet having a battery which is adapted to generate the high voltage and output to the high voltage subnet, and which has at least two battery units with Einzelspannungsabgriffen, which are guided to the coupling unit, wherein the coupling unit is set up to selectively connect the battery units to the low voltage subnet, it is provided that the one battery unit connected to the low voltage subnet et, which has the highest state of charge.

The method according to the invention has the advantage that a state is established during operation in which the battery units have approximately the same state of charge. In particular, this ensures that the cells age evenly, i. for example, have a same internal resistance and a same capacity. The supply of the low voltage subnet changes from a battery unit to that battery unit which has a correspondingly higher state of charge than the battery unit currently used to supply the low voltage subnet.

Another advantage is that the requirements for the starting operations in the low-voltage subnetwork are always met, since in each case that battery unit is used, which currently has the best performance. Since the supply of the low voltage subnet is superimposed on the charging and discharging in the high voltage subnet and the low voltage subnetwork unidirectional, is ensured by the inventive method that always the battery unit with the highest state of charge is discharged faster or is charged slower than the other sub-batteries. This results in a symmetrization of the charge states of the sub-batteries result.

According to a preferred embodiment, a change of the connected battery unit occurs when a threshold value of a state of charge difference of the battery units is exceeded. This ensures that at the same or similar state of charge of the battery units no quick, constant change from one battery unit to the next, followed by a return change, as soon as the unused battery unit has the highest state of charge. Particularly preferably, the threshold value of the charge state difference of the battery units is a defined value between 0.5% and 20%, preferably between 1% and 5%, particularly preferably about 2%.

According to a preferred embodiment, the changeover of the connected battery unit takes place in that in a first step a current-carrying battery unit is switched off and thereafter in a second step a selected further battery unit is switched on. An advantage arises in particular when the coupling unit has reverse blocking switches, which are conductive circuit breakers. Due to the operation of the reverse blocking switch would be connected to the higher potential of the two battery units during the switching phase with simultaneous actuation of the switch, the positive pole of the low voltage subnet, and the negative pole of the electrical system with the lower potential of the two battery units, resulting in an increased voltage. The proposed switching strategy prevents short-term greater voltages from being supplied to the low-voltage subnetwork than the specification of the low-voltage subnetwork permits. The advantageous switching concept also prevents a brief increase in the low voltage during switching operations in the coupling unit used. In conjunction with a buffer device, which is designed, for example, as a capacitor in the low-voltage subnetwork, the voltage dip in the low-voltage subnetwork is further advantageously limited.

The voltage dip in the low-voltage subnetwork can be further advantageously reduced if the switchover occurs at such times when the on-board electrical system current is as low as possible. This can be done, for example, by evaluating a signal for the on-board electrical system current and dependent control of the switch of the coupling unit. In addition, synchronization with a consumer management system can also be made to accommodate high power consumers, such as consumer electronics. Switch off heating systems for a short time without sacrificing comfort, in order to enable switching of the battery units without significant voltage drop.

The invention also proposes a computer program according to which one of the methods described herein is performed when the computer program is executed on a programmable computer device. The computer program can be, for example, a module for implementing a device for operating an electrical system or a module for implementing a battery management system of a vehicle. The computer program may be stored on a machine-readable storage medium, such as on a permanent or rewritable storage medium, or in association with a computing device, such as a portable storage such as a CD-ROM, DVD, Blu-ray Disc, USB stick, or the like memory card. Additionally and alternatively, the computer program may be provided for download on a computing device, such as on a server or a cloud server, for example via a data network, such as the Internet, or a communication link, such as a telephone line or a wireless link.

According to the invention, a battery management system (BMS) is also provided which has means for carrying out one of the described methods for operating one of the described electrical systems. In particular, the battery management system has a unit which is set up to determine the charge state of the battery units, and in particular the battery unit with the highest charge state, and a unit which is set up to control the coupling unit so that battery units are selectively connected to the low voltage subnet, in particular the one with the highest state of charge.

According to a further embodiment, the battery management system comprises a further unit, which is set up to detect the exceeding of a threshold value of a charge state difference of the battery units, in order subsequently to perform a change of the connected battery unit by means of the coupling unit.

According to the invention also discloses a motor vehicle, with an internal combustion engine and a vehicle electrical system described above.

Advantages of the invention

The invention provides a low-cost vehicle electrical system with a lithium-ion battery system for vehicles comprising a high voltage subnet, for example with a 48 volt generator, and a low voltage subnet, for example with a 14 volt starter, and a boost recuperation system with unidirectional Have supply to the low voltage subnet. This can be compared to known systems a potential-separating DC / DC converter omitted, and the lead-acid battery. The Boost recuperation system, with a suitable design compared to BRS systems currently under development, can store significantly more energy and thereby recover more electrical energy in the system during longer braking or downhill driving.

The system is also characterized by its low volume, low weight and long life. Due to the multiple redundant designed low voltage subnetwork, there is a higher availability of the system, which allows high performance requirements even during cold starts.

The proposed method according to the invention comprises an operating strategy which enables the supply of the low-voltage subnetwork and provides electrical energy during start-up operations. The storage of electrical energy is optimized so that as much electrical energy can be recovered in a braking operation and the battery can be charged with the highest possible performance.

In addition, the storage of electrical energy is allowed in Rekuperationsvorgängen and provided as much electrical energy during boost operations. Cold starts and start / stop operations can be carried out with high performance, if necessary with rapid repetition.

Brief description of the drawings

Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description. Show it

1 a low-voltage electrical system according to the prior art,

2 a vehicle electrical system with a high-voltage sub-network and a low-voltage sub-network and a unidirectional, potential-separating DC / DC converter,

3 a vehicle electrical system with a high voltage subnet and a low voltage subnetwork and a bi-directional, potential-separating DC / DC converter,

4 an electrical system with a high-voltage sub-network and a low-voltage sub-network and a unidirectional, galvanically non-separating DC / DC converter,

5 a coupling unit according to an embodiment of the invention,

6 the coupling unit off 5 in an exemplary operating state,

7 the coupling unit off 5 during an exemplary switching operation, and

8th reverse blocking switch.

1 shows a wiring system 1 According to the state of the art. When starting an internal combustion engine is via the electrical system 1 from a starter battery 10 a tension a starter 11 provided which starts the internal combustion engine (not shown) when, for example, by a corresponding starter signal a switch 12 is closed. When the combustion engine is started, it drives an electric generator 13 which then has a voltage of about 12 Volt generated and via the electrical system 1 the various electrical consumers 14 in the vehicle. The electric generator 13 It also charges the starter battery loaded by the starting process 10 back up.

2 shows a wiring system 1 with a high voltage subnet 20 and a low voltage subnet 21 and a unidirectional, isolated DC / DC converter 22 , which is a coupling unit between the high voltage subnet 20 and the low voltage subnet 21 forms. The electrical system 1 may be an electrical system of a vehicle, in particular a motor vehicle, transport vehicle or forklift.

The high voltage subnet 20 is for example a 48-volt electrical system with an electric generator 23 which is operable by an internal combustion engine (not shown). The generator 23 is formed in this embodiment, in response to a rotational movement of the engine of the vehicle to generate electrical energy and in the high voltage subnet 20 feed. The high voltage subnet 20 further includes a battery 24 , which may be formed, for example, as a lithium-ion battery and which is adapted to output the necessary operating voltage to the high voltage subnet. In the high voltage subnet 20 are further load resistors 25 arranged, which may be formed for example by at least one, preferably by a plurality of electrical consumers of the motor vehicle, which are operated with the high voltage.

In the low voltage subnetwork 21 , which on the output side to the DC / DC converter 22 is located, there is a starter 26 which is set up, a switch 27 to close to start the engine, as well as an energy storage 28 which is set up, for example, the low voltage 14 Volt for the low voltage subnet 21 provide. In the low voltage subnetwork 21 are more consumers 29 arranged, which are operated with the low voltage. The energy storage 28 includes, for example, galvanic cells, in particular those of a lead-acid battery, which in the fully charged state (state of charge, SOC = 100%) usually has a voltage of 12.8 volts. When the battery is discharged (state of charge, SOC = 0%), the energy storage device 28 unloaded a terminal voltage of typically 10.8 volts. The vehicle electrical system voltage in the low-voltage subnetwork 21 is in driving mode, depending on the temperature and state of charge of the energy storage 28 , approximately in the range between 10.8 volts and 15 volts.

The DC / DC converter 22 is input side with the high voltage subnet 20 and with the generator 23 connected. The DC / DC converter 22 is the output side with the low voltage subnet 21 connected. The DC / DC converter 22 is configured to receive a DC voltage received on the input side, for example a DC voltage with which the high voltage subnet is operated, for example between 12 and 48 volts, and to generate an output voltage which is different from the voltage received on the input side, in particular to produce an output voltage which is smaller than the voltage received on the input side, for example 12 V or 14 V.

3 shows a wiring system 1 with a high voltage subnet 20 and a low voltage subnet 21 , which by a bidirectional, potential-separating DC / DC converter 31 are connected. The illustrated electrical system 1 is essentially like that in 2 formed electrical system, wherein the generator is integrated in the high voltage subnet and for the energy transfer between the sub-board networks 20 . 21 a DC / DC converter 31 is used, which is designed to isolate potential. In both subnets 20 . 21 are also batteries 24 . 28 and consumers 25 . 29 arranged as related to 2 described. In essence, that differs in 3 illustrated system by the involvement of the starter. While in the in 2 illustrated system of the starter 26 in the low voltage subnetwork 21 is arranged and thereby the DC / DC converter 22 unidirectional for energy transport from the high voltage subnetwork 20 into the low voltage subnet 21 can be designed at the in 3 illustrated architecture a starter generator 30 in the high voltage subnet 20 used. In this case, the DC / DC converter 31 Bidirectional running, so the lithium-ion battery 24 if necessary via the low voltage subnetwork 21 can be loaded. The jump start of the low-voltage vehicle then takes place via the low-voltage interface and the DC / DC converter 31 ,

4 shows a wiring system 1 with a high voltage subnet 20 and a low voltage subnet 21 For example, an electrical system 1 a vehicle, in particular a motor vehicle, transport vehicle or forklift. The electrical system 1 is particularly suitable for use on vehicles with a 48 volt generator, a 14 volt starter and a boost recuperation system.

The high voltage subnet 20 includes a generator 23 which is operable by an internal combustion engine (not shown). The generator 23 is configured to generate electrical energy in response to a rotational movement of the engine of the vehicle and into the high voltage subnet 20 feed. In the high voltage subnet 20 are load resistors 25 arranged, which may be formed for example by at least one, preferably by a plurality of electrical consumers of the motor vehicle, which are operated with the high voltage.

The high voltage subnet 20 also includes a battery 40 , which may be formed for example as a lithium-ion battery and which is adapted to output the operating voltage of 48 volts the high voltage subnet. The lithium-ion battery 40 has at a rated voltage of 48 volts, preferably a minimum capacity of about 15 Ah in order to store the required electrical energy can.

The battery 40 has several battery units 41-1 . 41-2 , ... 41-n on, taking the battery units 41 associated with several battery cells, which are usually connected in series and partly in addition to each other in parallel to the required power and energy data with the battery 40 to achieve. The individual battery cells are, for example, lithium-ion batteries with a voltage range of 2.8 to 4.2 volts.

The battery units 41-1 . 41-2 , ... 41-n are single voltage taps 42-1 . 42-2 , ... 42-n + 1 assigned, via which the voltage of a coupling unit 33 is supplied. For a series connection of the battery units 41-1 . 41-2 , ... 41-n , as in 4 shown are the Einzelspannungsabgriffe 42 between the battery units 41 arranged, as well as at the ends of the battery 40 one each. With a number of n battery units, this results in n + 1 taps 42 , Due to the additional single voltage taps 42 is the lithium-ion battery 40 in several battery units 41-1 . 41-2 , ... 41-n subdivided, which can also be referred to as part batteries within the scope of the invention. The single voltage taps 42 are chosen so that the battery units 41 each have a voltage level with which the low voltage subnet 21 , ie the 14 volt electrical system, can be supplied. The single voltage taps 42 the battery units 41 be like in 4 represented, the coupling unit 33 fed. The coupling unit 33 has the task, at least one of the battery units 41 the battery 40 on the low voltage subnet 21 to switch to its operation or support.

The coupling unit 33 couples the high voltage subnet 20 with the low voltage subnet 21 and provides the output side of the low voltage subnet 21 the necessary operating voltage ready, for example, 12 V or 14 V. The structure and operation of the coupling unit 33 be related to the 5 to 7 described.

The low voltage subnet 21 includes the low-voltage consumers 29 , which are designed for example for operation at 14 V voltage. The low voltage subnet 21 also has the starter 26 on which is set up, the switch 27 to start the combustion engine.

wobei I_max der maximale Bordnetzstrom ist, der während der Umschaltvorgänge im Bordnetz fließen kann, t_umschalt die Zeitdauer, während welcher keine Batterieeinheit 41 für die Versorgung bereit steht, und ΔU_max die maximal zulässige Veränderung der Bordnetzspannung während des Umschaltvorgangs.To supply the starter in the low voltage subnetwork, especially for a cold start of the vehicle, is in the low voltage subnet 21 another energy storage 28 available. The energy storage 28 , for example, a battery, can briefly deliver very high currents and relieves the lithium-ion battery 40 in the starting phases. Specifically, the effects of the known weakness of lithium-ion batteries that they can not deliver high currents at low temperatures, are through the use of energy storage 28 in the 4 shown reduced system. In the event that the energy storage 28 is designed as a double-layer capacitor, the starting currents can also take place in high total number over the entire life of the battery and even at individual startups possibly several times in succession, ie in case of unsuccessful start attempt, after the recharging of the power storage. In this way, a system can be realized, the very high availability of electrical energy in the low voltage subnet 21 has and can deliver very high power for a short time, that is optimized for high performance. The high performance storage 28 also fulfills the purpose that overvoltages when switching the battery units 41 be avoided. Used as energy storage 28 a capacitor is used, its dimensioning is preferred:

where I _{max is} the maximum on-board electrical system current that can flow during the switching operations in the electrical system, t _switch the time during which no battery unit 41 is ready for the supply, and ΔU _max the maximum permissible change in the vehicle electrical system voltage during the switching process.

According to one embodiment, it is provided that the lithium-ion battery 40 the supply of closed circuit consumers who are consumers 25 . 29 are shown, takes over when the vehicle is parked. For example, it may be provided that in this case the requirements of the so-called airport tests are met, wherein after six weeks of service life, the vehicle is still bootable and the battery during the service life, the quiescent currents of the low-voltage consumers 29 in the low voltage subnetwork 21 provides, for example, an anti-theft alarm system is powered.

This in 4 illustrated electrical system may further comprise a battery management system (BMS) (not shown). The battery management system comprises a control unit, which is set up, measurement data on temperatures, voltages provided, discharged currents and charge states of the battery 40 or the battery units 41 to record and process and from this, for example, statements about the health of the battery 40 hold true. The battery management system in this case comprises a unit which is set up, the coupling unit 33 so that these are the battery units 41 selectively in the low voltage subnetwork 21 can switch on.

5 shows a coupling unit 33 , which is designed as a unidirectional, galvanically non-isolated DC-DC converter (DC / DC converter). The coupling unit 33 includes reverse blocking switches 44 . 45 which have the property that in a state "on" they allow current to flow in only one direction and in a second state "off" they can absorb a blocking voltage of both polarities. This is an essential difference to simple semiconductor switches, such as IGBT switches, as they can not pick up reverse voltage due to their intrinsic diode. Due to the dependence on the current flow direction are in 5 drawn two different types of switches, namely RSS_l 45 and RSS_r 44 , which do not differ in their production, but are installed only with different polarity. An example of the closer construction of the reverse-blocking switch 44 . 45 is related to 8th described.

In the coupling unit 33 become the single voltage taps 42 the battery units 41 each at branch points 43 branched and each one of the different reverse blocking switch RSS_l 45 and RSS_r 44 fed. The reverse blocking switch RSS_l 45 be the output side of the coupling unit 33 with the positive pole 52 interconnected, and the reverse blocking switch RSS_r 44 be the output side of the coupling unit 33 on the negative pole 51 connected.

6 shows the supply of the low voltage subnetwork 21 by way of example from the battery unit 41-2 about the assigned taps 42-2 and 42-3 , From the positive pole 52 leads the current path 61 via a reverse blocking switch RSS_l 45-i over a branch point 43-i over the voltage tap 42-2 to the through-connected battery unit 41-2 , and from there via the through-connected battery unit 41-2 arranged voltage tap 42-3 over the branch point 43-j via another reverse blocking switch RSS_r 44-i to the negative pole 51 , At the first branch point 43-i also connects to another reverse blocking switch RSS_r 44-j , Since this is formed backward blocking, but no current can flow here. In the case of a common MOSFET switch, this would be passing backwards so that the current path is not across the battery pack 41-2 would lead, but via the switch RSS_r 44-j , The same applies to the second branch point 43-j which in turn leads to a blocking reverse blocking switch RSS_l 45-j leads, so that here also no current flow is possible.

The voltage level of the high voltage subnetwork 20 based on the mass of the low voltage subnetwork 21 depends on which of the battery units 41 is switched on. In none of the operating states, however, does any of the potentials have an amount that exceeds a voltage limit equal to the sum of the high voltage and the low voltage, that is, approximately 62 volts for a 48 volt mains and a 14 volt mains. However, negative potentials can occur with respect to the ground of the low voltage subnetwork.

The operation of the high voltage generator 23 is independent of the operation of the coupling unit 33 and the supply of the low voltage subnet. In the through-connected battery unit 41 that the low voltage subnet 21 supplied, results in a superposition by the low voltage sub-network power and possibly from the generator 23 in the entire lithium-ion battery 40 fed charging current (generator operation) or by the entire lithium-ion battery 40 removed discharge current (motor operation). As long as the permissible limits of the battery cells, eg the maximum permissible discharge current of the cells, are not exceeded, these processes can be considered independently of each other. So the low voltage subnet 21 is safely supplied, exactly one of the battery units 41 via the associated switches 44 . 45 the coupling device 33 switched on. Due to the multiple redundant supply of the low voltage subnet 21 can be built with the architecture presented a system that has a very high availability of electrical energy in the low voltage subnet.

7 shows a switching operation by means of the coupling unit 33 by way of example of the battery unit 41-1 on the battery unit 41-n , Before switching, a first current path leads 71 via a first reverse blocking switch RSS_l 45-i , about first voltage taps 42-1 . 42-2 , the first battery unit 41-1 assigned via a second reverse blocking switch RSS_r 44-i to the negative pole 51 , After switching, the current path leads 72 via a second reverse blocking switch RSS_l 45-k , about voltage taps 42-n . 42-n + 1 , which is the nth battery unit 41-n are assigned and via another reverse blocking switch RSS_r 44-k to the negative pole 51 ,

• – Die Umschaltung erfolgt so, dass die Schalter der aktuell Strom führenden Teilbatterie, im dargestellten Beispiel die Batterieeinheit 41-1, zuerst abgeschaltet werden und nachdem die Schalter der bisher Strom führenden Teilbatterie keinen Strom mehr führen, werden die Schalter der Teilbatterie, welche die Versorgung des Niederspannungsteilnetzes übernehmen sollen, eingeschaltet. Das beschriebene Prinzip wird auch als „Break-before-Make“ bezeichnet.

To change the reverse-blocking switch 45-i . 44-i turned off and the other reverse-blocking switch 45-k . 44-k switched on. Would the coupling unit 33 the switching commands for the switches 45-i . 44-i . 45-k . 44-k get synchronous, it would be due to the operation of the backward barrier switch, the positive pole 52 of the low voltage subnet during the switching phase of the circuit breaker connected to the higher potential of the two sub-batteries and the negative terminal 51 during the switching phase with the lower potential of the two sub-batteries, ie in the example with the negative terminal of the battery unit 41-n , In this way, a substantially greater voltage would be applied to the low-voltage subnetwork in the short term than the specification of the low-voltage subnetwork permits. In the example shown in FIG 6 would the low voltage subnet 21 because of the series connected battery units 41 the sum of the partial voltages of the total battery are provided at short notice. To overcome these surges avoid is when switching the coupling unit 33 proceed as follows:

• - Switching takes place in such a way that the switches of the current-carrying sub-battery, in the example shown, the battery unit 41-1 , are turned off first and after the switches of the previously leading part battery no longer carry electricity, the switches of the sub-battery, which should take over the supply of the low voltage subnet, are turned on. The principle described is also referred to as "break-before-make".

Considering an optimized operating strategy for the electrical system 1 with the illustrated series connection of the battery units 41 the following considerations are made. It is assumed that with uniformly aged cells, the internal resistance and the capacity of the cells at the same reference conditions, ie substantially the same temperature and the same state of charge, are approximately equal.

The maximum achievable power is limited by uniformly aged cells by that cell with the lowest charge state.

The maximum removable energy is limited by evenly charged cells by the cell with the lowest state of charge.

The maximum allowable load performance is limited by the cell with the highest state of charge for evenly aged cells.

The maximum deliverable energy is limited by evenly aged cells by the cell with the highest state of charge.

Since the battery system in a boost recuperation system should be able to store as much energy as possible during a braking process at the same time as being able to support a boost process as well as possible, the requirement can be derived from this the battery units 41 and the cells therein must all have the same state of charge as possible in order to meet the requirements as well as possible.

In addition to the requirements for the high voltage subnet 20 The system also has requirements for the startup processes in the low-voltage subnetwork 21 posed. To meet these requirements by combining the high-performance energy storage 28 and the lithium-ion battery 40 be as well met, is preferably that battery unit 41 used to supply the low voltage subnetwork, which has the highest state of charge at a given time.

The requirements for the selection of the switching states of the coupling unit 33 can be met with the following operating strategy: The supply of the low voltage subnetwork 21 always takes place from that part battery 41 , which currently has the highest state of charge. Since the supply of the low-voltage subnetwork is superimposed on the charging and discharging processes in the high-voltage subnetwork and the low-voltage sub-network power supply takes place unidirectionally, this selection rule ensures that the sub-battery 41 with the highest state of charge is discharged faster or is charged slower than the other battery units 41 , This results in a symmetrization of the charge states of the sub-batteries result.

So that at the same state of charge of the battery units 41 not a very fast change from a battery unit 41 set to the next, a threshold for the difference .DELTA.SOC _{switching of} the charge states is introduced, for example, a difference .DELTA.SOC _switching with a defined value between 0.5% and 20%, preferably between 1% and 5%, more preferably about 2% must be exceeded, so that the supply of the low voltage subnet 21 from a battery unit 41 on the battery unit 41 changes, which has a correspondingly higher state of charge than currently for the supply of the low voltage subnet 21 used battery unit 41 , Switching in the supply is always on that battery unit 41 , which currently has the highest state of charge, and the switching takes place when the current for the supply of the low voltage subnet 21 through-connected battery unit 41 has a state of charge, which is at least ΔSOC _switching less than the state of charge of that battery unit 41 with the highest charge level.

8th shows a possible construction of reverse blocking switches 44 . 45 , The forward direction is indicated by I. A reverse-blocking switch RSS_r 44 includes, for example, an IGBT, MOSFET or bipolar transistor 101 and a diode connected in series 103 , In 8th is a MOSFET 101 which is an intrinsic diode shown with 102 having. Which to the MOSFET 101 series connected diode 103 is opposite to the direction of the intrinsic diode 102 of the MOSFET 101 poled. The reverse blocking switch RSS_r 44 lets the current in the forward direction I and locks in the opposite direction. The reverse blocking switch RSS_l 45 corresponds to the RSS_r 44 , is only installed with the reverse polarity, so that the forward and reverse directions are reversed. The switches RSS_l 45 , RSS_r 44 are characterized in particular by a barely noticeable delay in the switching operations, ie allow a very short switching time. By means of a suitable drive circuit, the time delay between switching off and switching on the switches can be set very precisely.

The invention is not limited to the embodiments described herein and the aspects highlighted therein. Rather, within the scope given by the claims a variety of modifications are possible, which are within the scope of expert action.

Claims (11)

Electrical system ( 1 ) for a motor vehicle, with a low voltage subnet ( 21 ) for at least one low-voltage consumer ( 29 ) and with a high voltage subnet ( 20 ) for at least one high voltage consumer ( 25 ) and an electric generator ( 23 ), the high voltage subnetwork ( 20 ) with the low voltage subnet ( 21 ) via a coupling unit ( 33 ), which is set up, the high voltage subnetwork ( 20 ) And the low voltage subnet ( 21 ), the high voltage subnetwork ( 20 ) a battery ( 40 ), which is designed to generate the high voltage and to the high voltage subnet ( 20 ) and the at least two battery units ( 41 ) with single voltage taps ( 42 ), which are connected to the coupling unit ( 33 ) are guided, characterized in that the coupling unit ( 33 ), the battery units ( 41 ) the low voltage subnet ( 21 ) selectively switch on. Electrical system ( 1 ) according to claim 1, characterized in that the selectively switchable battery units ( 41 ) are each designed to provide the low voltage. Electrical system ( 1 ) according to one of the preceding claims, characterized in that the coupling unit ( 33 ) reverse-blocking switches ( 44 . 45 ) having. Electrical system ( 1 ) according to one of the preceding claims, characterized in that the low-voltage subnetwork ( 21 ) at least one energy store ( 28 ), which is configured to generate the low voltage and to the low voltage subnet ( 21 ). Electrical system ( 1 ) according to one of the preceding claims, characterized in that the low-voltage subnetwork ( 21 ) a starter ( 26 ) having. Method for operating a vehicle electrical system ( 1 ) for a motor vehicle, the on-board network ( 1 ) a low voltage subnet ( 21 ) for at least one low-voltage consumer ( 29 ) and a high voltage subnet ( 20 ) for at least one high voltage consumer ( 25 ) and an electric generator ( 23 ), wherein the high voltage subnetwork ( 20 ) with the low voltage subnet ( 21 ) via a coupling unit ( 33 ), which is set up, the high voltage subnetwork ( 20 ) And the low voltage subnet ( 21 ), the high voltage subnetwork ( 20 ) a battery ( 40 ), which is designed to generate the high voltage and to the high voltage subnet ( 20 ), and the at least two battery units ( 41 ) with single voltage taps ( 42 ), which are connected to the coupling unit ( 33 ) are guided, wherein the coupling unit ( 33 ), the battery units ( 41 ) the low voltage subnet ( 21 ), characterized in that the battery unit ( 41 ) the low voltage subnet ( 21 ) is switched on, which has the highest state of charge. A method according to claim 6, characterized in that a change of the connected battery unit ( 41 ) when exceeding a threshold value of a state of charge difference of the battery units ( 41 ) he follows. A method according to claim 6 or 7, characterized in that a change of the connected battery unit ( 41 ) is carried out by, in a first step, a current-carrying battery unit ( 41 ) and thereafter in a second step a selected further battery unit ( 41 ) is switched on. Battery management system for carrying out one of the methods according to one of claims 6 to 8, having a unit for determining the state of charge of battery units ( 41 ) and with a unit for controlling a coupling unit ( 33 ) for the selective connection of battery units ( 41 ) on the basis of determined charging states of the battery units ( 41 ).  A computer program adapted to perform any of the methods of any one of claims 6 to 8 when the computer program is executed on a programmable computer device. Motor vehicle with an internal combustion engine and a vehicle electrical system ( 1 ) according to one of claims 1 to 5.

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